Apparatus for pyrolyzing hydrocarbons

ABSTRACT

A HYDROCARBON RAW MATERIAL IS PYROLYZED TO LOWER UNSATURATED ALIPHATIC HYDROCARBONS BY MIXING THE RAW MATERIAL WITH HOT COMBUSTIONS GASES AT A RATE SUFFICIENT TO HEAT THE MIXTURES ABOVE THE PYROLYIZING TEMPERATURE. THE ENDOTHERMERIC REACTION IS PERFORMED IN A POROUS TUBE WHILE OXYGEN IS BEING FORCED INTO THE TUBE THROUGH THE WALL TO SUPPLY THE THERMAL ENERGY CONSUMED AND TO MAINTAIN THE PYROLYSIS TEMPERATURE BY OXIDATION OF A PORTION OF THE PYROLYSIS PRODUCT, PARTICULARLY HYDROGEN. THE REACTION MIXTURE IS THEN QUICKLY COOLED.

United States Patent US. Cl. 23-277 4 Claims ABSTRACT OF THE DISCLOSUREA hydrocarbon raw material is pyrolyzed to lower unsaturated aliphatichydrocarbons by mixing the raw material with hot combustion gases at arate sufficient to heat the mixture above the pyrolyzing temperature.The endothermic reaction is performed in a porous tube while oxygen isbeing forced into the tube through the wall to supply the thermal energyconsumed and to maintain the pyrolysis temperature by oxidation of aportion of the pyrolysis product, particularly hydrogen. The reactionmixture is then quickly cooled.

This application is a continuation-in-part of our copending applicationSer. NO. 674,570, filed on Oct, 11, 1967, now abandoned.

This invention relates to the pyrolysis of hydrocarbons to unsaturatedaliphatic hydrocarbons having fewer carbon atoms, and particularly to apyrolyzing method and to apparatus for performing the method.

It is known to mix a hydrocarbon raw material with gaseous combustionproducts in order quickly to raise the raw material to a temperature atwhich pyrolitic decomposition takes place. The ensuing reaction isendothermic so that the temperature of the raw material quickly reachesa maximum during mixing with the combustion gases, but then drops. Yet,it is known that the most desirable pyrolysis products are obtained bymaintaining the temperature or by even gradually increasing thetemperature during pyrolysis. The afore-described known process cannotachieve optimum results and the formation of carbonaceous solids insubstantial amounts cannot be avoided.

It is also known to raise the temperature of the raw material while thesame passes through a tube having a porous wall. Either a hot combustiongas or oxygen is forced into the tube under pressure. If oxygen is soadmixed to the hydrocarbon, combustion of the latter provides the heatfor reaching pyrolysis temperature and the formation of carbonaceousdeposits on the reaction 'vessel is prevented. If hot combustion gas oroxygen is supplied through the permeable walls of a conduit holding theflowing raw material, the latter can be heated only relatively slowly.It dwells for relatively long periods in zones where the temperature issufliciently below the proper pyrolysis temperature to favor theformation of undesirable by-products. Moreover, it has not beenpractical to build such permeable, tubular conduits of a size useful inindustrial production. A sizable portion of the space in the reactionchamber actually serves as a preheating chamber. If hot combustion gasesare forced into the chamber through the wall, the thermal lossessignificantly aflect the cost of operation.

Attempts at overcoming the difliculties outlined above have beenhampered by the high cost of materials capable of withstanding thetemperatures necessary for py- 3,563,709 Patented Feb. 16, 1971 rolysisand the even higher cost of shaping such materials.

A primary object of the invention is the provision of a continuouspyrolysis method for a hydrocarbon raw material in which the temperaturecan be controlled at will along the stream of reactants, moreparticularly, the raw material is heated almost instantaneously to thepyrolysis temperature, and the thermal energy consumed by theendothermic reaction is replenished as needed to provide constant oreven rising temperature through the reaction zone. Another object is theprovision of reliable and practical apparatus for performing the method.

In the method of the invention, a stream of fuel is burned with anoxygen-bearing gas to produce a stream of hot combustion gas. The latteris mixed with a stream of the hydrocarbon raw material to be pyrolyzedat a rate sufficient to raise the temperature of the mixture so producedto the pyrolysis temperature of the raw material. The mixture is thenpassed through a conduit having a porous wall While at pyrolysistemperature, whereby a major portion of the raw material is thermallydecomposed in the conduit. An additional amount of oxygen bearing gas isintroduced inward of the conduit through the porous wall at a ratesuflicient to supply the thermal energy consumed by the endothermicpyrolysis reaction, whereby the temperature is at least substantiallymaintained, but may be increased by oxidation of a portion of thereaction products. The remainder of the products is then withdrawn fromthe conduit.

The apparatus employed includes the burner required for burning thefuel, a reaction chamber having a wall of permeable material, and asource of hydrocarbon raw material. A mixing device is interposedbetween the burner and the reaction chamber and is connected to the rawmaterial source for receiving the combustion gas and the raw material,mixing the same, and discharging the mixture so produced into thereaction chamber. A pressure chamber is in contact with a face of theaforementioned wall outside the reaction chamber and means are providedfor feeding an oxygen bearing gas to the pressure chamber. The reactionchamber has an outlet for discharge of a reaction mixture formedtherein, and a cooling device is provided for cooling the dischargedreaction mixture.

Further objects, additional features, and many of the attendantadvantages of this invention will readily be appreciated as the samebecomes better understood by the following detailed description ofpreferred embodiments when considered in connection with the appendeddrawing in which:

FIG. 1 shows a pyrolysis apparatus of the invention in side elevation,and partly in section; and

FIG. 2 shows a modified element for use in the apparatus of FIG. 1.

Referring initially to FIG. 1, there is seen a sectional tower whosetopmost element is a combustion chamber 1 flanged to a Venturi mixer 2.The latter is mounted atop an upright tubular vessel 3 whose cavity isdivided by a coaxial, cylindrical wall 4 of porous material into acentral reaction chamber 5 and an annular pressure chamber 15. Thebottom section of the tower which supports the vessel 3, the mixer 2,and the combustion chamber 1, is a cooling chamber 6 having a wideoutlet 7 in its curved vertical wall. The chamber 1, 5, and 6 and thediverging-converging passage of the mixer 2 jointly form a straightvertical conduit.

The otherwise closed top wall of the combustion chamber 1 is separatelysupplied with fuel and oxygen through supply lines 8, 9, and the lengthof the flame and the tem-.

perature of the combustion gas can be controlled in a known manner by asteam inlet 10 on the lower portion of the combustion chamber near themixer 2.

, A fluid hydrocarbon raw material is admitted to the throat or mixingchamber of the Venturi mixer 2 by a pipe 11. A flanged nipple 12 on thevessel 3 admits oxygen under pressure to the chamber 15. A pipe 13communicating with the cooling chamber 6 near the top of the latterpermits a cooling fluid to be introduced into the chamber 6 above theoutlet 7. The bottom flange 14 of the chamber 6 may be apertured in aconventional manner, not shown, to permit discharge of pyrolysisproducts not "passingthrough the outlet 7 and of an excess, ofliquidcoolingflui'difemployed. i

It will be understood that the apparatus is further equipped withcontrol valves in the several supply lines for proper adjustment ofprocess variables, and with indicating or recording instruments formeasuring flow rates of materials entering the illustrated apparatus andfor indicating temperatures wherever of interest.

' The combustion chamber 1, the mixer 2 and the cooling chamber 6 arelined with refractory material in a conventional manner. The wall 4 ismade of sintered spherical particles of phosphorbronze having a nominalcomposition of 92% copper and 8% tin, and a solidus temperature of 880C. Other materials which have been used'successfully include a similarbronze wall prepared by sintering short length of wire, walls ofsintered nickel and stainless steel, and sintered ceramic materials suchas alumina, zi'rconia, mullite, or cermets consisting mainly of'aluminaor chromium oxide and Cr, Mo, Co, Was the metallic constituent. It ispreferred to prepare the porous wall 4 by sintering, but other methodsof construction may be resorted to.

Hydrogen or a gas rich in hydrogen content is the preferred fuel whichis admitted to the combustion chamber 1 through the supply line 8. It isburned with a stoichiometrically equivalent amount of oxygen dischargedfrom the line 9. The gaseous residue recovered from the work-up of thepyrolysis products is usually a suitable fuel and may be recycled t thecombustion chamber 1.

Any gas containing elementary oxygen may be employed for combustion ifcommercially pure oxygen is not available or if the resulting dilutionof the product is acceptable. Atmospheric air or air enriched withoxygen 'may thus be employed.

" The temperature of the combustion gas can reach as steam through theinlet 10.

N The hot gas is mixed in the throat of the Venturi mixer 2 with thehydrocarbon raw material that isto be pyrolyz ed and which is initiallyin the liquid state. The temperature of the hydrocarbons is raisedalmost instan taneous ly to the desired pyrolysis temperature bysuitable control of the feed rates. Typically, the reaction temperatureis 750 C. for the preparation of propylene and ethylene as thepredominant pyrolysis products, and somewhat higher if it is desired toprepare mainly ethylene aiid acetylene, the necessary conditions ofpyrolysis being well known among those skilled in the art and notdifferent in the method of this invention from the usual operatingconditions. A

The period during which the raw material isheated through thetemperature range below the pyrolysis temperature is extremely short,and the percentage of undesired products known to be generated at thelower temperatures by polymerization, dehydrogenation, or cracking isminimal. It is further reduced if the temperature in the reactionchamber is controlled to rise in the direction'of fluid flow.

Thermal energy is supplied to the stream of material in the chamber 5 bypartial combustion of the pyrolysis products with secondary oxygensupplied through the porous wall 4 from the pressure chamber 15.Hydrogen, methane, and carbon monoxide in the mixture are preferentiallyoxidized to maintain the initial pyrolysis temperature, or to raise thetemperature of the gaseous stream for further pyrolysis of compounds ofrelatively low much as 3000 C. and may be adjusted by introducingmolecular weight formed in the initial stage of pyrolysis.

The oxygen or oxygen bearing gas employed in the secondary combustionenters the pressure chamber through inlet 12 at relatively lowtemperature, and thus protects the wall 4 against the high temperaturesprevailing elsewhere in the reaction chamber 5. The flow of gas throughthe pores of the wall 4 is rapid enough to prevent the deposition ofcarbon on the inner wall surface which would impede further entry ofsecondary oxygen.

The reaction mixture is quickly cooled in the chamber 6, typically toabout 500 C., by a fluid coolant introduced through the pipe 13. Anysuitable and available process fluid may be employed as coolant, and itmay be liquid or gaseous. Water in the liquid form or as steam may beemployed, but liquid or gaseous hydrocarbons have also been employed. Anexcess of liquid coolant, if any, is withdrawn through the bottom flange14 whereas the gaseous pyrolysis products together with combustionproducts and volatile coolant are withdrawn from the illustratedapparatus through the outlet 7 for recovery of thermal energy andfractionation in a conventional manner.

The temperature in the several axial zones of the reaction chamber 5 maybe controlled more precisely by axially dividing the pressure chamber 15and by individually controlling the admission of oxygen to thecompartments so formed. FIG. 2 illustrates a different method ofcontrolling the temperature distribution in a combustion chamber 5 3Oradially bounded by a porous wall 4' which flares conically in adirection from the Venturi mixer 2 toward the cooling chamber 6. Themixer and cooling chamber are not shown in FIG. 2, and it will beunderstood that the apparatus of FIG. 2 is identical with thatillustrated in FIG. 1 as far as not specifically shown in the drawing.

Because of the conical shape of the wall 4, its permeability to oxygenentering from the pressure chamber 15 through an axial unit length ofthe wall increases in a direction away from the combustion chamber 1.Conversely, the ultimate flowv rate of the pyrolysis mixture in thechamber 5' is lower than in the cylindrical chamber 5 if the initialflow rate was the same. It is therefore easier to maintain an increasingtemperature in the flowing pyrolysis mixture in the chamber 5 than inthe chamber 5.

Obviously, the shape of the reaction chamber in the pyrolysis chamber ofthe invention may be modified otherwise to adapt it to specificprocessing conditions. It has been found, however, that one of theadvantages of the apparatus illustrated is its great versatility, andits ability to operate successfully over the entire range of conditionsnormally required for pyrolysis of hydrocarbon raw materials tocompounds having shorter carbon chains, more specifically lower alkenesand lower alkynes.

The following examples are further illustrative of the method ofinvention as performed in apparatus of the type illustrated:

EXAMPLE 1 A laboratory reactor of the type shown in FIG. 1 was used forpyrolysis of a gasoline fraction boiling between 80 and 180 C. Theporous wall 40f the reactor had an internal diameter of 40 mm., andother dimensions of the combustion chamber 1, the Venturi mixer 2, andthe vessel 3 may be read from the drawing which is substantially toscale with respect to elements 1, 2, 3, 4.

The gasoline entered the mixer 2 through the pipe 11 at a rate of 5 kg.per hour and a temperature of 500 C. For each kilogram of rawhydrocarbon stock, the combustion chamber was supplied with 0.415 cubicmeter of a fuel gas consisting of 42% hydrogen, 38% carbon monoxide, and20% methane, and having a net heating value of 4.792 cal. per m It willbe understood that all percentage ,values are by volume unless statedotherwise, and that absolute values of gas volume relate to measurementsreduced to standard conditions of temperature and pressure.

Oxygen was supplied to the combustion chamber 1 at a rate of 0.440 m.and to the pressure chamber 15 at a rate of 0.140 m. per kg. ofhydrocarbon stock. The dwell time of the reaction mixture in the tube 4was 0.01 to 0.001 second, and the temperature in the tube had an averagevalue of approximately 1,000 to 1,100 C., and increased by about 200 C.in the direction of gas flow. The pressure in the tube 4 wasapproximately 7 p.s.i.g., and the pressure differential across the wall4 was approximately 20 mm. Hg.

The effiuent gas contained, on a dry basis, 20.7% ethylene, 3.9%acetylene, 4.5% propylene, and 28.9% hydrogen, the remainder beingcarbon monoxide, carbon dioxide, methane, and smaller amounts of ethane,propane and butane. The material recovered by condensation per kilogramof raw gasoline feed consisted of 0.418 kg. ethylene, 0.074 acetylene,and 0.139 propylene.

EXAMPLE 2 The reactor of Example 1 was supplied with the same gasolinefraction at a rate of 5 kg. per hour. The combustion chamber wassupplied, per kilogram of hydrocarbon stock, with 0.480 m. fuel gas and0.510 ml oxygen while 0.162 m. oxygen were fed to the pressure chamber15.

The temperature in the tube 4 varied from 1,500 C. near the Venturimixer 2 to 1,700 C. near the cooling chamber 6. The dwell time in thepyrolysis zone was approximately 0.001 to 0.0001 second and the pressureabout 7 p.s.i.g. The pressure differential across the porous Wall was 25mg. Hg.

For each kilogram of gaseoline fed to the reactor, 0.24 kg. ethylene and0.23 kg. acetylene were recovered, the remainder of the reactionproducts consisting essentially, in the order of decreasing quantities,of hydrogen, carbon monoxide, carbon dioxide, methane, ethane, propane,and butane.

The effect of higher operating temperature on the average chain lengthof the pyrolysis product is evident. Other variations in the operatingconditions of the reactor may obviously be resorted to, and theirresults are predictable.

We claim:

1. A pyrolysis apparatus comprising, in combination:

(a) sources for the supply of a portion of the total amount of oxygenbearing gas to be used and of fuel;

(b) combustion means for burning said fuel to a combustion gas;

(c) a reaction chamber having a wall of permeable material;

(d) a source of hydrocarbon raw material;

(e) mixing means interposed between said combustion means and saidreaction chamber for receiving said combustion gas and said rawmaterial, for mixing the received combustion gas with said raw material,and for discharging the mixture so produced into said reaction chamber;

(f) a pressure chamber in contact with a face of said wall outside saidreaction chamber;

(g) means for feeding the remaining portion of oxygen bearing gas tosaid pressure chamber, said reaction chamber having an outlet fordischarge of a reaction mixture formed therein; and

(h) cooling means for cooling the discharged reaction mixture.

2. In the apparatus according to claim 1, said wall of permeablematerial forming said reaction chamber, extending longitudinally, andbeing cylindrically shaped, a vessel constituting said pressure chamber,said vessel surrounding, and being coextensive and eccentric with, saidreaction chamber.

3. An apparatus as set forth in claim 1, wherein said combustion means,said mixing means, and said reaction chamber constitute respectiveportions of a continuous conduit, said conduit flaring in cross sectionin said combustion chamber in a direction away from said mixing means.

4. An apparatus as set forth in claim 3, wherein said wall of saidcombustion chamber is frustoconical.

References Cited UNITED STATES PATENTS 1,808,168 6/1931 Hopkins 26068332,174,288 9/1939 Klein et a1. 260683.3 2,387,731 10/1945 Allen 2606802,790,838 4/1957 Schrader 260683 3,161,695 12/1964 Gofiinet 2606793,375,288 3/1968 Rosset 260669 DELBERT E. GANTZ, Primary Examiner C. E.SPRESSER, J R. Assistant Examiner U.S. Cl. X.R.

